Elemental items (Primals, Eternals, etc.) can be a good source of money, because they are always in demand on the AH. See which mobs you will most benefit from farming, then set out for a few hours. In the process you will also most likely collect significant vendor trash, and may get other profitable item drops as well. Note, however, that older elemental drops usually lose profitability compared to newer ones, i.e. Primal Water sold well to level 70 players in BC, but sells far less well to level 80 players in WotLK, because level 80 gear requires Eternal this'n'that.
The neutral auction house can become one of your best friends. Other players are constantly checking both factions' auction houses to find an appropriate price for the good they demand. Often times, goods are cheaper on the alliance because they have more people, thus, the supply increases as demand decreases and price goes down. If you are horde, feel free to transfer gold to your alliance toon through the neutral auction house in order to buy them item and bring it back to the horde side where it is more expensive. The 15%, as of patch 3.3.3 (down from 30%) is well worth the extra 50% or more you can easily make on one item alone. Blizzard decreased the neutral cut for a reason to 15%, that reason is to get trade facilitated between the horde and alliance. Make sure you take advantage of this! Tips when trading between factions:
With Mining, from the moment you take the skill you can make good money selling stacks of the bars or ores in the Auction House. Blacksmiths, Engineers and Jewelcrafters are interested in the bars, while Jewelcrafters and even other miners may want to buy the ores. Make sure to check the relative prices of ore versus bars before smelting. As your skill increases, so does your earning potential. At the maximum level you may smelt Titansteel for a daily fee.
Some players even turn this into their profession by systematically "plundering" vendors in the game world and then selling the items on the auction house at a significant markup. The reason why this works (even for items which are on unlimited supply at vendors) is, that many players don't want to spend time traveling to specific vendors to get hold of a recipe or skill book. They would rather pay a slightly higher price at their local auction house. In some sense, they use the auction house as a "super market" or "convenience store". So it is completely reasonable and legitimate to be the supplier for this convenience store and make money out of it.
Cool the whlelping spot that has caused so much controversy int he gold farming community. I still think spot is mentionable because it's really the only reliable spot to pull out a crimson whelping. Most of you are probablly thinking this farm is old and beat to death, and you are right. BUT that does not take away the fact that the whelping does drop highest from this farm. If you can get it from the slavemasters your looking at anywhere from 25,000g and above. A cool nifty old spot, that has survived the test of time.
First of all, the value of transmog is decided by rarity and how they look. Some items like Minenarbeithut der Tiefen or Pendel der Verdammnis (they can drop in Uldaman) are worth millions of gold. They are worth so much, because they have a really small chance to drop. Other items are bought well because they look good. Best example in this case is the Glorious set, its legplates Glorreichen Beinplatte reach a 100.000 Gold value easily.